A fuel cell is an electrochemical device that produces an electromotive force by bringing the fuel (typically hydrogen) and an oxidant (typically air) into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode where it reacts electrochemically in the presence of the electrolyte to produce electrons and cations in the first electrode. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the second electrode. Simultaneously, an oxidant, such as oxygen or air is introduced to the second electrode where the oxidant reacts electrochemically in presence of the electrolyte and catalyst, producing anions and consuming the electrons circulated through the electrical circuit; the cations are consumed at the second electrode. The anions formed at the second electrode or cathode react with the cations to form a reaction product. The first electrode or anode may alternatively be referred to as a fuel or oxidizing electrode, and the second electrode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the two electrodes are, respectively, as follows:H2→2H++2e−½O2+2H++2e−→H2O
The external electrical circuit withdraws electrical current and thus receives electrical power from the cell. The overall fuel cell reaction produces electrical energy as shown by the sum of the separate half-cell reactions written above. Water and heat are typical by-products of the reaction.
In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds to the electrodes, while cooling is provided either by the reactants or by a cooling medium. Also within the stack are current collectors, cell-to-cell seals and insulation, with required piping and instrumentation provided externally of the fuel cell stack. The stack, housing, and associated hardware make up the fuel cell module.
Various parameters have to be monitored to ensure the proper operation of a fuel cell stack and evaluate the performance thereof. These parameters include the voltage across each fuel cell in the fuel cell stack, hereinafter referred to as cell voltage, and the internal resistance of each fuel cell.
Issues arise when designing systems for monitoring these parameters, such as portability, fuel cell applicability, measurement variety, resolution, automation and cost. These issues have been addressed, to some extent, in the assignee's co-pending U.S. patent application Ser. No. 09/672,040 and No. 10/109,003, that describe a self-contained, portable apparatus/system for measuring fuel cell impedance during fuel cell testing and a related method. The system comprises a CPU, frequency synthesizer, a fuel cell, a load bank and measurement and acquisition circuitry. The CPU receives input parameters from a software program and sends the parameters to a signal generation device, which produces an AC waveform with a DC offset that is used to remotely program a load bank. The load bank draws current from the fuel cell. The voltage across the fuel cell and the current through the fuel cell are measured by voltage and current sensing circuitry, then digitized and averaged by an oscilloscope or A/D converter. The recorded data is sent to the CPU where the AC phase lead or lag is calculated. Numerous outputs can then be displayed by the invention, including real impedance, imaginary impedance, phase difference, leading component, lagging component, current magnitude, voltage magnitude and applied AC voltage.
However, the inventions of the earlier applications have limited application in the measurement of fuel cell impedance in fuel cell stacks during actual operation of the fuel cell stack (“in the field” operation). Further, a scheme for measuring the internal resistance of individual fuel cells within a fuel cell stack in a real-time manner is not detailed in the previous patent application.
In order to measure cell voltages, differential voltage measurement is required at the two terminals (i.e. anode and cathode) of each fuel cell. However, since fuel cells are connected in series, and typically in large number, the voltages at some terminals will be too high for any currently available semiconductor measuring device to directly measure. For example, for a fuel cell stack consisting of 100 cells with each cell voltage at 0.95 V, the actual voltages on the negative terminal (cathode) of the top cell will be 94.05 V (i.e. 0.95*100−0.95). As such, the voltage exceeds the maximum allowable input voltage of most current differential amplifiers commonly used for measuring voltage.
The assignee's co-pending U.S. patent application Ser. No. 09/865,562 provides a solution for this problem. This patent application provides a system for monitoring cell voltages of individual fuel cells in a fuel cell stack during testing; the contents of U.S. patent application Ser. Nos. 09/865,562, 09/672,040 and 10/109,003 are hereby incorporated by reference. The system of patent application Ser. No. 09/865,562 comprises a plurality of differential amplifiers, a multiplexer, an analog to digital converter, a controller and a computer. Each of the differential amplifiers reads the voltages at two terminals of each fuel cell. The analog to digital converter reads the output of the differential amplifiers via the multiplexer, which provides access to one of these differential amplifiers at any given time. The digital output of the analog to digital converter is then provided to the computer for analysis. The computer controls the operation of the analog to digital converter and the multiplexer. However, the voltage monitoring system in this patent application only measures the DC voltage across individual fuel cells. In contrast, in the aforementioned U.S. patent application Ser. No. 09/672,040, which described a method and system used in fuel cell testing, the measurement of impedance involves applying both AC and DC voltages across a complete fuel cell stack, whether this is a single fuel cell or a stack of many fuel cells.
Thus, there is still need for a system that is suitable for measuring internal resistance of individual fuel cells within a fuel cell stack, especially a stack consisting of a large number of fuel cells, during actual use of the fuel cell “in the field”, as opposed to a controlled testing environment used for fuel cell testing purposes.